Overall, 8% (21 birds) of breeding Herring Gulls showed black on the primary coverts. Black was more frequent in males than females (males 12% (n = 146); females 3% (n = 128); Pearson χ2=6,99; P=0,008; df=1). This difference may be due to an age-sex interaction with respect to colonisation, as males return to the colony to occupy territories at younger age than females (Vercruijsse, 1999). For birds of known age (n = 33), the presence of black on the primary coverts is age-related in Herring Gull (b = -1.393, r2 = 0.408, P = 0.014; df = 1; table 2).

We scored 54 adult birds of known age in 6cy-14cy. 41 birds (75%), in all nine age-classes, were found showing neatly demarcated black pigmentation. 62 breeding LBBG in our sample are considered to be ‘sub-adult birds’ showing primary coverts with extensive black markings, pale brown tips and diffuse borders. 666 birds show limited black markings which are neatly demarcated; we consider such birds to be ‘mature adults’ (see plate 3).

Plate 3. Two example primary covert patterns to illustrate the division between ‘sub-adult’ and ‘adult’ Lesser Black-backed Gulls in our analysis. Left: the greater primary coverts in sub-adult LBBG oHZ show extensive dark centres and diffuse borders. Also note the dark centres on the secondaries and brown-black P9 and P10. Right: LBBG yET with neatly demarcated black markings, which are considered to be still possible in ‘adult’ birds. In fact, LBBG yET was ringed as pullus in June 1995, and now in its 12th calendar year.

.

LBBG

HG

Age

No

Yes

n:

No

Yes

n:

4 cy

.

100

13

.

100

2

5 cy

14

86

8

40

60

5

6 cy

20

80

5

67

33

3

7 cy

17

83

7

80

20

5

8 cy

8

92

14

100

.

5

9 cy

50

50

4

100

.

3

10 cy

57

43

7

100

.

1

11 cy

50

50

6

100

.

3

12 cy

.

100

5

.

.

.

13 cy

33

67

3

100

.

1

14+ cy

.

100

3

100

.

5

Whole sample

25

75

962

92

8

286

Table 2: Black pigmentation on primary coverts in known-age Herring and Lesser Black-backed Gulls (retrapped birds excluded). Values are % for their own age-class. Age in calendar years (CY).